Low-cost, wire-bonding processes for copper interconnect and copper packaging technologies are a critical issue that needs to be resolved for copper interconnection technologies to rapidly penetrate the consumer goods sector of the electronics market. Using the present technology, gold wire may not be directly attached to a final copper metal level because of the poor reliability of the copper-to-gold wire-bonding metallurgy. In addition, wire-bond pads containing copper surfaces that are exposed to the environment are unacceptable because exposed copper surfaces are prone to corrosion.
One process, directed to circumventing the issues associated with the copper-to-gold bonding, is the use of aluminum as a final metal film to contact the gold wire bond. The use of an aluminum film requires, however, an additional photomask step and an additional patterning step, which is typically reactive ion etching. These two additional steps, together with the associated pre-cleaning and photoresist removal steps, require additional processing materials, additional time, and additional expenses.
The prior art process which uses an aluminum film to provide contact between copper and gold bonding wires is best described as follows. After the final metal cooper structure is formed on the surface to be bonded, a dual dielectric film is deposited over the copper surface. This deposition is followed by a polyimide layer formed over the dielectric film. The polyimide serves as a further insulator. The polyimide and underlying passivating dielectric are patterned and etched using reactive ion etching. In this manner, a section of the final copper metal film is exposed. This exposed section will be used to provide contact to the wire-bonding gold. Exposed surfaces of copper film are undesirable for providing direct contact to gold wire bonds.
Therefore, at this point in the conventional process, an aluminum film is added. A dual metal layer is deposited as follows. First, a barrier film, which isolates the copper and aluminum metals from each other, is deposited. The deposition of the barrier material is not a selective deposition process. As such, the barrier material contacts the exposed copper surface and also covers the patterned dielectric and polyimide films. Next, an aluminum film is formed over the entire surface of the barrier layer. This dual layer (barrier layer and aluminum) material must then be patterned and subsequently etched. A photolithographically sensitive film is used to form a pattern of this dual layer metal film. After the films are removed by etching, and the photolithographically sensitive film is removed, the gold wire bond may be connected to the aluminum film which is contacted to the copper film through the barrier material.
Another, less expensive, alternative to aluminum wire bonding over copper may be the use of an electroless deposition process to form compounds such as CoP and NiP as contact layers for gold wire bonding. In this process, after a pattern has been created in the insulating layers to expose the portion of the final copper film which is to be bound (as above), a layer of CoP is selectively deposited over the exposed film region. This CoP film acts a copper barrier. The CoP layer is then passivated with electroless NiP to prevent the oxidation of the CoP. The NiP is plated selectively over the CoP barrier film. The NiP is then selectively covered with gold using immersion or electroless plating techniques. After the immersion or electroless gold film has been selectively deposited, the structure is ready for wire bonding to the gold wire-bonding line.
A simpler structure could be formed by eliminating the CoP barrier layer and depositing NiP directly over the exposed copper surface. A major shortcoming of this approach is that, above 300.degree. C., the NiP film interacts with the exposed copper to produce a bond pad metallurgy with a resistance which is too high for wire-bonding applications. Therefore, what is needed in the art is a procedure which uses the selectively deposited NiP film but does not require the CoP barrier layer, yet produces a bond pad metallurgy with acceptably low resistance values.